Greens and veggies are wonderful additions to a birds main diet if fed in moderation. Make sure the birds are on a good formulated poultry diet first, and extras as a small treat.
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As the others have said, fruits and veggies are great for them, but you still want them to eat their regular feed. I'd let them peck at the veggies for a short time and then remove them from the coop. If you aren't already, maybe you can start composting with the whatever the chickens aren't eating. I would eliminate the scratch from their diet. If they're already getting treats daily or almost daily they don't need any extra treats.